Three phase AC motors are generally simpler, more reliable and more efficient than single phase AC motors. In addition to three phase AC motors, much high-power industrial equipment requires three phase AC power. The load of such equipment can be inductive, capacitive or resistive.
Three phase AC power is generally supplied to industrial areas. However, only single phase AC power is available to most residential and rural areas. The single phase AC power available in most residential and rural areas is provided by a step down transformer connected to a high voltage line and, in the United States, is normally supplied as about 240 volts or 480 volts at 60 Hz between the first and second input lines.
For areas where three phase AC power is not available, various solutions have been used to convert single phase power to three phase power. For motor load applications, static and rotary converters are a low cost solution. These static and rotary converters, however, have a high percentage of voltage unbalance and low efficiency. Inverters, including AC drives, convert the entire single phase AC input voltage into DC voltage then invert the DC voltage into AC three phase output voltage. Such inverters are generally more complex and expensive than static and rotary converters. The output voltages of such inverters consist of pulse-width-modulated (PWM) signals with high harmonic content, so that the applications of these inverters are limited to inductive motor loads unless additional expensive filtering circuits are used.
U.S. Pat. No. 6,297,971 to Meiners discloses a digital phase converter that converts a single phase input voltage into a three phase output voltage without converting the whole input AC voltage into DC voltage. The converter disclosed converts 240V AC single phase voltage into balanced three phase AC, and can be used for inductive, capacitive and resistive loads with about 1% output voltage imbalance. U.S. Pat. No. 6,731,525 to Meiners discloses another digital phase converter that converts a single phase input voltage into a three phase output voltage without converting the whole input AC voltage into DC voltage.